If you go into Settings>sound & display>ringer volume and uncheck the box that is checked another volume bar will show up which controls notifications. You can then slide that all the way to the left and they sound will be shut off. Hope that helps.

Try Setting Profiles, which you can use to change the volume settings overnight and return them to the defaults in the morning. The free version only allows one active rule at a time, but one is all you need to do that. It's worth the $3.00 to buy the full version.

Check out the free app, "AudioManager Widget" by Smart Android Apps. It has a nice UI that gives you individual sliders for all of your audio options in one place. This is prolly the easiest solution to your issue.